Provision of a rigid sail onto the deck of an engine-driven ship has recently been proposed to effectively utilize wind force for the navigation of the ship from the point of view of energy saving, and some ships provided with rigid sails are practically in use.
One of the industrialized rigid marine sails is shown in FIGS. 1 to 4 (hereinafter referred to as the "prior art 1").
The above-mentioned prior art 1 is described below with reference to the drawings. In FIGS. 1 to 4, 1' is a mast vertically secured onto a deck 2' of a ship in such a manner that the lower end portion of the mast 1' is inserted into a mast support 3' fixed onto the deck 2', and the front face of the mast 1' forming part of the sail surface; 4' is a mast rotation mechanism for rotating the mast 1' around the vertical axis thereof, the mast rotation mechanism 4' comprising a gear 5' horizontally fixed to the lower end portion of the mast 1' and a motor 6' secured onto a side of the mast support 3' for rotating the gear 5'; 7' are two pivot shafts one each provided on the left and the right sides of the mast 1' substantially in parallel with the mast 1'; 8'A and 8'B are a left sail portion and a right sail portion which are pivotally secured respectively to the left and the right sides of the mast 1' substantially in parallel therewith through the respective pivot shafts 7'; 9' is a drive mechanism for symmetrically pivoting the left and the right sail portions 8'A and 8'B relative to the mast 1' around the respective pivot shafts 7' between the deployed position and the folded position, the drive mechanism 9' comprising a movable rod 11 secured to the mast 1' through a plurality of guide members 10 in parallel with the mast 1', vertically moving up and down, a lifting means 12 which comprises a hydraulic cylinder and other parts for vertically moving up and down the movable rod 11, and plural pairs of vertically spaced connecting rods 13 provided at portions of the left and the right sail portions 8'A and 8'B near the respective ends on the sides facing the pivot shafts 7'; ends of the plural pairs of connecting rods 13 being connected respectively to the left and the right sail portions 8'A and 8'B through respective swivel bearings 14A, and the other ends of the plural pairs of connecting rods 13 being connected respectively to the movable rod 11 through respective another swivel bearings 14B.
Now, operation of the rigid marine sail of the above-mentioned prior art 1 is described. To pivot the left sail portion 8'A and the right sail portion 8'B from the deployed position shown in FIGS. 1 and 2 to the folded position shown in FIGS. 3 and 4, the lifting means 12 is actuated to lower the movable rod 11, whereby the other ends of the plural pairs of connecting rods 13 are lowered down until the plural pairs of connecting rods 13 become substantially vertical. Thus, the left and the right sail portions 8'A and 8'B are pivoted to the folded position symmetrically relative to the mast 1' around the pivot shafts 7'. To pivot the left and the right sail portions 8'A and 8'B from the folded position to the deployed position, on the other hand, the lifting means 12 is actuated to raise the movable rod 11, whereby the other ends of the plural pairs of connecting rods 13 are raised up until the connecting rods 13 become substantially horizontal. Thus, the left and the right sail portions 8'A and 8'B are pivoted to the deployed position symmetrically relative to the mast 1' around the pivot shafts 7'.
The above-mentioned prior art 1 however involves the problems as described below. As described above, the left sail portion 8'A and the right sail portion 8'B are pivoted by the other ends of the plural pairs of connecting rods 13 vertically lowered down by means of the movable rod 11. The only force contributing to the pivoting motion of the left and the right sail portions 8'A and 8'B is the force acting horizontally, i.e., in a direction perpendicular to the pivot shafts 7' of the left and the right sail portions 8'A and 8'B from among the forces transferred from the movable rod 11 through the plural pairs of connecting rods 13 to the left and the right sail portions 8'A and 8'B. This horizontal force F is reduced to F.sub.o, F.sub.1 and then F.sub.2 according as the left and the right sail portions 8'A and 8'B approach the folded position, as shown in FIG. 5. In contrast, the vertical force F' transferred through the plural pairs of connecting rods 13 to the left and the right sail portions 8'A and 8'B becomes larger to F'.sub.1 and then F'.sub.2 according as the left and the right sail portions 8'A and 8'B approach the folded position. At the moment when the left and the right sail portions 8'A and 8'B reach the folded position, therefore, an excessive vertical force F' acts on the left and the right sail portions 8'A and 8'B, and as a result, the left and the right sail portions 8'A and 8'B may be damaged.
A conceivable method for solving this problem is to increase the mechanical strength of the left and the right sail portions 8'A and 8'B. This however leads to a larger ship weight and higher shipbuilding costs. Another conceivable solution is, as shown in FIG. 6, to secure the left and the right sail portions 8'A and 8'B to the mast 1' with a longer distance L between the pivot shafts 7' and thus to increase the ratio of the vertical force F' to the horizontal force F transferred to the left and the right sail portions 8'A and 8'B. More specifically, in the case shown in FIG. 5, the ratio F'.sub.2 /F.sub.2 of the vertical force F'.sub.2 to the horizontal force F.sub.2 at the moment when the left and the right sail portions 8'A and 8'B reach the folded position is about 4.5, whereas, in the case shown in FIG. 6, the ratio F'.sub.2 /F.sub.2 is about 2.5. If the left and the right sail portions 8'A and 8'B are secured to the mast 1' with a longer distance L between the pivot shafts 7', therefore, an excessive vertical force F' does not act on the left and the right sail portions 8'A and 8'B even when the left and the right sail portions 8'A and 8'B reach the folded position, thus permitting prevention of damage to the left and the right sail portions 8'A and 8'B. However, when the left and the right sail portions 8'A and 8'B are thus secured to the mast 1' with a longer distance L between the pivot shafts 7' , the left and the right sail portions 8'A and 8'B would have an increased area between the end faces thereof on the side of the pivot shafts 7' (hereinafter referred to as the "front end area") when the left and the right sail portions 8'A and 8'B reach the folded position, resulting in an increased air resistance acting onto the front end area when the left and the right sail portions 8'A and 8'B reach the folded position, thus impairing stable navigation of the ship. This inconvenience is particularly serious when the ship runs against a head wind.
Our experiment shows that, when the left and the right sail portions 8'A and 8'B are pivoted to the deployed position with the use of the above-mentioned drive mechanism 9', the ratio of the distance between the pivot shafts 7' to the overall width of the sail cannot be reduced to below about 18% in view of the above-mentioned ratio F'/F of the vertical force F' to the horizontal force F.
Now, another prior art (hereinafter referred to as the "prior art 2") disclosed in European Patent Provisional Publication No. EP 0 064 107 A1 is described below. FIG. 7 is a plan view illustrating a rigid marine sail of the prior art 2 in the deployed position. As shown in FIG. 7, a pivot shaft 7' is secured to a vertical mast 1' in front thereof with a space from the mast 1' in parallel with the mast 1' through plural pairs of fittings 15, and a left sail portion 8'A and a right sail portion 8'B pivotally engage at respective ends thereof with the pivot shafts 7' through plural pairs of blackets 16. In front of the pivot shaft 7', a semi-cylindrical shrouding plate 18 is secured to the pivot shaft 7' in parallel with the pivot shaft 7' through a plurality of fixing rods 17. Notches (not shown) are formed on the shrouding plate 18 for access of the plural pairs of blackets 16. The shrouding plate 18 covers the front end area of the left and the right sail portions 8'A and 8'B when the left and the right sail portions 8'A and 8'B are pivoted to the folded position. The left and the right sail portions 8'A and 8'B are symmetrically pivoted around the pivot shaft 7' relative to the mast 1' by a drive mechanism 9' similar to that for the rigid marine sail of the above-mentioned prior art 1. More particularly, the left and the right sail portions 8'A and 8'B are symmetrically pivoted between the deployed position and the folded position around the pivot shaft 7 relative to the mast 1' by moving up and down a movable rod 11 secured to the mast 1' through a plurality of guide members 10 in parallel with the mast 1'. In FIG. 7, 13 are plural pairs of connecting rods connecting the left and the right sail portions 8'A and 8'B with the movable rod 11 through swivel bearings 14A and 14B fitted to the both ends of the connecting rods 13.
The rigid marine sail of the prior art 2 described above also involves a problem similar to that in the rigid marine sail of the above-mentioned prior art 1.
There is therefore a demand for a rigid marine sail in which, when the left sail portion 8'A and the right sail portion 8'B are pivoted to the folded position thereof, a vertical force F' does not act on the left and the right sail portions 8'A and 8'B, and in which it is possible to secure the left and the right sail portions 8'A and 8'B to a mast 1' so that the front end area is minimized when the left and the right sail portions 8'A and 8'B are pivoted to the folded position, but such a rigid marine sail is not as yet proposed.